First Statewide Abundance Estimate of the Florida Manatee

This new abundance survey is a benchmark achievement in monitoring Florida manatees.

First Statewide Abundance Estimate of the Florida manatee

The FWC has accomplished a key goal of its Manatee Management Plan with the publication of results from the first statewide abundance estimate of the Florida manatee in the journal, Biological Conservation. A primary conservation goal of the plan was to “implement peer-reviewed and statistically sound methods to estimate the manatee population and monitor trends”.

The findings represent a significant improvement over the traditional survey, commonly called the “synoptic survey” approach. Results from synoptic surveys provide only a minimum number of manatees known to be alive using warm water and winter habitats on a particular census day. The inability to account for manatees not seen during the fly over (related to weather and water conditions, and manatee behavior) results in counts that vary widely across surveys and are consequentially of limited utility. The new abundance survey is a benchmark achievement in monitoring Florida manatees. The new survey design accounts for key sources of bias and variation and provides an estimate of the Florida manatee population. Reliable estimates can be used to track population changes over time and as part of population projection models to provide valuable feedback to conservation managers.

Designing a new method for estimating manatees has been challenging because manatees occur over large landscapes and are often in near shore habitats that makes it difficult to apply traditional statistically sound survey methods. To meet this challenge, an innovative approach was designed, tested, and vetted with experts. This approach is based on a random sampling design and combines multiple sources of information. A combination of a double-observer protocol (i.e., multiple observers in each plane), repeated passes, and detailed diving behavior data were used to account for imperfect detection of animals.

Results:

The newly published estimate uses data collected from February 28 to March 22, 2011 along Florida’s west coast and from March 5 to 13, 2012 along Florida’s east coast.

The estimate of abundance for the period 2011 and 2012 with 95% confidence (CI) was 6,350 (95% 5,310-7,390)

West: 2,790 (95% 2,160-3,540)

East: 3,560 (95% 2,850-4,410)

What does this range mean? It tells us about the uncertainty surrounding the estimate. The point estimate of 6,350 by itself is of limited usefulness because it does not reveal the uncertainty associated with this estimate. We do not have a good sense for how far this sample mean is from the true population mean without the perspective of the confidence interval. If our statistical assumptions are correct, the confidence interval tells us that we are 95% confident that the true population lies between the lower and upper bounds.

Limitations and other considerations:

This new approach is based on statistical models, as with any models it relies on assumptions that if violated can lead to biased estimates. In addition, because this is a new method there are several points that must be taken into consideration when interpreting the results.

1) Although our manatee population estimate is an improvement over past information, we believe we have underestimated the abundance for several reasons:

a) Groups of manatees are more easily detected by observers than are individual manatees. However, in this model, groups of manatees and individual manatees are treated the same way (have the same probability of being detected by an observer);

b) We accounted for observer’s ability to detect a manatee based on water clarity (turbidity) but not for variation in sea state or sea surface roughness (in the future we hope to find a field method for conducting our visibility study in higher sea states that could then be used to update abundance estimates);

c) Although it is theoretically possible to obtain estimates for regions with zero counts, we removed these regions from the estimation of overall abundance because including them would have led to technical numerical issues. Thus, our inference is restricted to regions in which sightings were reported.

2) The surveys to estimate abundance are four times more expensive than the traditional synoptic surveys. The abundance surveys are also more challenging logistically because they require more aircraft and pilots, more time to coordinate and implement and more trained observers. This is why we conducted the surveys over a period of two years (see Results).

3) The new estimate is on a statewide scale, but biologically there is little evidence of manatee movements between coasts, suggesting relative demographic isolation and hence a rationale for an east-west division for management purposes. Genetic information shows some variability between coasts, but not complete genetic isolation.

Now that we have a new method, will it replace the traditional synoptic survey? It is not logistically possible to fly both coasts in any given winter using the new approach, therefore we will assess the frequency of conducting future surveys using the new methods in light of statutory requirements and information needs. It may be possible to meet these information needs using other population measures such as adult survival rates from photo-identification studies in combination with available abundance survey estimates within a short-term projection model framework. Annual population estimates resulting from this modeling approach may be used to quantify short-term population changes. This framework would allow us to strategically determine a schedule for conducting the new survey design which is more costly and labor intensive than the traditional survey. Finally, this new methodology is amenable to improvement over time, since additional visibility studies and other enhancements can help improve the models in the future.

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